1. Field of the Invention
The present invention relates to a semiconductor wafer provided with a dot mark and to a method for making the dot mark. More particularly, the present invention relates to a semiconductor wafer provided with a dot mark, in which a surface of the semiconductor wafer is free from contamination which arises when the dot mark is formed using laser beam and in which the dot mark retains its form even after various steps for manufacturing the semiconductor, and the visibility is ensured throughout all steps. Further, the present invention relates to a dot marking method using laser beam.
2. Description of the Related Art
Conventionally, it has been disclosed in Japanese Patent Application Laid-Open Nos. 59-17235 and 61-95990 that in order to avoid adhesion of splashes, which are produced by the vaporization of a semiconductor wafer, to a surface of the semiconductor wafer in a laser marking step, a protective film is formed in advance on the surface of the semiconductor wafer prior to the marking and is irradiated with laser beam to form a mark on both the protective film and the surface of the semiconductor wafer.
For instance, according to the laser marking method disclosed in Japanese Patent Application Laid-Open No. 59-17235, a transmission protective film such as a silicon oxide film is formed on a whole or a part of the surface of the semiconductor wafer and is irradiated with laser beam with a high energy density from above to form a mark on the surface of the semiconductor wafer while breaking the protective film. Then, the protective film is removed. According to this marking method, the splashes in the process adheres to the surface of the protective film and the adhered splashes are removed together with the protective film. Thereafter, only simple washing is required. In Japanese Patent Application Laid-Open No. 61-95990, an organic film is used in place of the above-mentioned silicon oxide film. In this laser marking method, also, all molten splashes of the wafer produced in the marking step adhere to the resin film in the molten state. Therefore, no splashes adhere directly to the wafer, and a high quality semiconductor wafer with a dot mark can be obtained by a subsequent removal of the resin film and washing.
Meanwhile, in a process for manufacturing a semiconductor, diverse and strict manufacturing conditions must be provided in each step. In order to control these conditions, a mark of, for example, numerals, characters or bar codes is dot-displayed. Therefore, the number of the semiconductor manufacturing steps are more than 100, in addition, many process steps, such as forming elements or flattening, are performed in each step. These steps include, for example, an application of resist, reduced protuberance of a pattern on the resist, development of a resist, or flattening by various films such as insulating or metal films to fill up gaps generated by copper wiring and the like.
On the other hand, the above marking by dots is not generally finished by only one marking and the necessary minimum history data is frequently marked in each manufacturing step to know the historical characteristics in each manufacturing step. However, the marking area on the semiconductor wafer is limited to a very small range, which limits the size and number of dots to be marked. Hence the size of marking area and the size and number of dots are defined in the SEMI standard. Generally, in the marking of this type, as disclosed, for example in Japanese Patent Application Laid-Open Nos. 6-301690 and 2-299216, identification codes such as a form name, a lot name and a wafer number of the manufactured semiconductor device are marked on blank areas in the vicinity of an orientation flat section of a semiconductor wafer or in the peripheral sections of the processed surface of integrated circuits of each chip.
The semiconductor wafer dot-marked in this manner is read as a variation in reflectance by the irradiation of the beam of He--Ne laser, or a variation in the oscillation of heat wave by the ordinary laser beam. Based on the information read in this manner, various manufacturing conditions in the subsequent manufacturing steps are set. Therefore, in the case where the above reading is not made correct and incorrect information is read, all wafers will become inferior products except for casual cases. The incorrect reading is mostly caused by blurred dot marks.
Therefore, as far as the portion to be dot-marked is a processed surface of an integrated circuit as described above, there is no guarantee that easiness of reading with respect to the dot mark can be kept even in the marginal area. In particular, in the case where the area in which the dot mark is formed is in the periphery section of a semiconductor wafer, since various steps for forming films and steps for partially removing films are repeated as described above, and the periphery section of wafer is, in particular, an area where holding and releasing are repeated, it is more difficult to take care of the surface than in other sections. Also, since in forming or removing the films as described above, the dot mark is buried due to the formed film, or the depth of the mark tends to be shallowed due to an excessive removal of the film, easiness of reading is reduced every time when the wafer goes through a plurality of steps of process.
On the other hand, according to the marking method disclosed in the above publications, the protective layer is broken by laser beam in marking in order to form a dot mark on the semiconductor water under the protective layer. Although splashes produced in marking can be prevented from adhering to the water surface, the protective layer is removed from the wafer every time when the marking is carried out. Therefore, a protective film must be newly formed in the next marking and this procedure is repeated over several times. A protective film is therefore newly formed also on the mark that has been formed by the last time every time a marking step is carried out. Hence the dot mark is buried due to the protective film or scraped when each film is removed whereby the shape of the mark is greatly changed. As the steps of process go on, it becomes more difficult to read the marks formed previously.